The Programmed Death 1 (PD-1) protein is an inhibitory member of the CD28 family of receptors, which also includes CD28, CTLA-4, ICOS and BTLA. PD-1 is expressed on the surface of activated B cells, T cells, and myeloid cells. PD-1 contains a membrane proximal immunoreceptor tyrosine inhibitory motif (ITIM) and a membrane distal tyrosine-based switch motif (ITSM). Although structurally similar to CTLA-4, PD-1 lacks the MYPPY motif that is critical for B7-1 and B7-2 binding. In addition, although CD28, ICOS and CTLA-4 (other members of the CD28 family) all have an unpaired cysteine residue allowing for homodimerization, PD-1 is believed to exist as a monomer, lacking the unpaired cysteine residue characteristic in other CD28 family members. The PD-1 receptor has two ligands, PD-ligand-1 (PD-L1) and PD-L2. The term “PD-L1” refers to the ligand of the PD-1 receptor also known as CD274 and B7H 1. PD-L1 is a 290 amino acid protein with an extracellular IgV-like domain, an extracellular IgC-like domain, a transmembrane domain and a highly conserved intracellular domain of approximately 30 amino acids. PD-L1 is constitutively expressed on many cells such as antigen presenting cells (e.g., dendritic cells, macrophages, and B-cells) and on hematopoietic and non-hematopoietic cells (e.g., vascular endothelial cells, pancreatic islets, and sites of immune privilege). The term “PD-L2” refers to the ligand of the PD-1 receptor also known as CD273 and B7-DC. PD-L2 has an extracellular IgV-like domain, an extracellular IgC-like domain, a transmembrane domain and an intracellular domain of approximately 30 amino acids in humans. PD-L2 has a more restricted expression than PD-L1, with its expression largely confined to hematopoietic cells including macrophages, dendritic cells, some B cell subsets and bone marrow-derived mast cells.
PD-1 functions as an immune checkpoint and works to prevent the activation of T-cells. PD-1 antagonists activate the immune system to attack tumors and have shown success in treating cancers, and in some instances, with less toxicity than other chemotherapeutic treatments. PD-1 antagonists can also be used in combination regimens with other chemotherapeutic agents. Currently approved PD-1 antagonists include anti-PD-1 antibodies, Opdivo® and Keytruda®, and anti-PD-L1 antibody, Tecentriq™.
There remains a need for additional antagonists of PD-1 for treatment of cancer and other diseases and disorders.